1. Field of the Invention
The present invention relates to a semiconductor light emitting device, an optical semiconductor device, a light emitting diode and a display device and, more particularly, a semiconductor light emitting device, an optical semiconductor device, a light emitting diode and a display device all using as a light emitting portion a wurtzite compound semiconductor such as GaN which is able to emit a light having a wavelength ranging over blue to ultraviolet.
2. Description of the Prior Art
In recent years, development of a short wavelength laser which has a wavelength in the range of blue to ultraviolet and used as an optical disk light source has been carried out extensively. As a blue laser light source, there have been optical devices using II-VI ZnSe system material and II-V GaN system material. The ZnSe system material has been ahead of the GaN system material in research of the semiconductor laser and further room-temperature continuous oscillation by means of the ZnSe system material has already been reported. However, since degradation easily occurs in the ZnSe system material in essential, the ZnSe system material has the problem in reliability and therefore it has not been put to practical use yet.
In contrast, after high luminance LED using GaN system material has been published several years ago, the GaN system material being excellent in the environmental resistance has been reconsidered and the researcher have been increased in number all over the world. The semiconductor laser which enables laser oscillation by the InGaN system material has been published by Nichia Chemical Ltd. in the year 1996.
Since the GaN system semiconductor used as the light emitting device may be formed of wurtzite compound semiconductor, it has been epitaxially grown on a hexagonal system sapphire substrate or 6H--SiC substrate, which has a similar crystal structure, by virtue of the MOVPE (metal organic chemical vapor deposition) method. Where a notation "H" of 6H--SiC denotes a crystal having six-fold symmetry, and a notation "6" denotes a crystal having a six phase synchronization structure in which atomic or molecular alignment are periodically formed in six phases.
Formation of the semiconductor laser using the hexagonal system sapphire substrate or 6H--SiC substrate will be made according to following procedures.
In the case of the sapphire substrate, for example, as shown in FIG. 24A, an n type Al.sub.0.1 Ga.sub.0.9 N cladding layer 143, an n type GaN light guide layer 144, a Ga.sub.0.9 In.sub.0.1 N active layer 145, a p type GaN light guide layer 146, and a p type Al.sub.0.1 Ga.sub.0.9 N cladding layer 147 are epitaxially grown via a GaN buffer layer 142 on a sapphire (0001) substrate 141 using (0001) plane as the principal plane virtue of the MOVPE method, and then part of the n type Al.sub.0.1 Ga.sub.0.9 N cladding layer 143 is exposed by etching. An n side electrode made of a Ti/Au electrode 148 is provided on an exposed surface of the n type Al.sub.0.1 Ga.sub.0.9 N cladding layer 143, and a p side electrode made of an Ni/Au electrode 149 is also provided on the p type Al.sub.0.1 Ga.sub.0.9 N cladding layer 147. Thus a semiconductor laser is completed via above steps.
On the contrary, in the case of the 6H--SiC substrate, as shown in FIG. 24B, an n type Al.sub.0.1 Ga.sub.0.9 N cladding layer 153, an n type GaN light guide layer 154, a Ga.sub.0.9 In.sub.0.1 N active layer 155, a p type GaN light guide layer 156, and a p type Al.sub.0.1 Ga.sub.0.9 N cladding layer 157 are epitaxially grown via an n type AlN buffer layer 152 on a (0001) Si face, i.e., Si face of a 6H--SiC (0001) substrate 151 by virtue of the MOVPE method. An n side electrode made of a Ti/Au electrode 158 is provided on a back surface of the 6H--SiC (0001) substrate 151, and a p side electrode made of an Ni/Au electrode 159 is also provided on the p type Al.sub.0.1 Ga.sub.0.9 N cladding layer 157. Thus a semiconductor laser is completed via above steps.
In such conventional light emitting devices, since (0001) faces of GaN system epitaxial layers 142 to 147 and 152 to 157 are grown in the &lt;0001&gt; direction of the sapphire (0001) substrate 141 or the 6H--SiC (0001) substrate 151, distortion in the plane becomes isotropic and therefore the Ga.sub.0.9 In.sub.0.1 N active layers 145, 155 still remain as uniaxial anisotropy. The &lt;0001&gt; direction is the c axis direction.
Next, an energy band structure of the GaN system semiconductor will be explained.
FIG. 25A shows a band structure of valence band of the GaN system semiconductor to which no distortion is applied. In FIG. 25A, HH (Heavy Hole) and LH (Light Hole) bands are nearly degenerated and a CH (Crystalline field split Hole) band is near HH and LH.
In the conventional light emitting device shown in FIG. 24A, since in-plane lattice constants of the n type Al.sub.0.1 Ga.sub.0.9 N cladding layer 143 to the p type Al.sub.0.1 Ga.sub.0.9 N cladding layer 147 are defined by a lattice constant of the n type Al.sub.0.1 Ga.sub.0.9 N cladding layer 143 directly over the GaN buffer layer 142, the n type GaN light guide layer 144 to the p type GaN light guide layer 146, all grown coherently, undergo compressive stress due to lattice mismatching and difference in thermal expansion coefficient.
Similarly, the device shown in FIG. 24B undergoes tensile stress, but compressive stress is imposed on the Ga.sub.0.9 IN.sub.0.1 N active layer 155 because of strong influence of distortion due to the lattice constant.
The inventors of the present invention have found the fact that optical gain of GaN is very high in contrast to conventional material and therefore this material is suitable for a surface emittingtype semiconductor laser.
Then, the surface emitting type semiconductor laser will be explained.
The semiconductor laser has an optical resonator (vertical resonator) which has a resonance axis along the direction vertical to the active layer formed on the substrate, and has a structure for emitting the light in the direction perpendicular to a surface of the active layer. Such surface emitting type semiconductor laser has such advantageous features in characteristic and manufacturing that threshold current is low because of a short length of the resonator, a two dimensional array of the semiconductor laser elements can be easily achieved, the number of elements per unit wafer area can be made large, device test can be conducted as the wafer, etc. Accordingly, the surface emitting type semiconductor laser for oscillating a short wavelength light has been developed for use in optical disk or short distance optical communication.
However, since a direction of the polarozation plane of the oscillation light is not fixed in the surface emitting type semiconductor laser in the prior art, in some cases the polarozation plane varied in use, otherwise kink appears in thee oscillation light output characteristic owing to variation in the polarozation plane. In such event, reading of the optical disk by virtue of polarization of light cannot be effected, and writing/reading by using the light cannot be effected stably, and stable communication cannot be achieved.
Therefore, it has been requested to fix and stabilize face orientation of the light oscillated from the surface emitting type semiconductor laser.
In the prior art, the active layer of the surface emitting type semiconductor laser has been deposited on the substrate using (001) plane of a zincblende crystal as the principal plane. Meanwhile, if the surface emitting type semiconductor laser in which the hexagonal semiconductor representative of GaN is used as the active layer is composed of the similar structure, it will be expected that the active layer is deposited on the substrate which uses (0001) plane perpendicular to the c axis of the hexagonal crystal as the principal plane. The conventional surface emitting type semiconductor laser having such structure will be explained hereinbelow.
FIG. 26 is a perspective view showing the surface emitting type semiconductor laser in the prior art and represents a basic structure of the surface emitting type semiconductor laser having the vertical resonator.
In FIG. 26, a substrate 101 is formed of a hexagonal crystal, e.g., sapphire and has the principal plane perpendicular to the c axis 104. Alternatively, the substrate 1 uses (001) plane of the zincblende crystal as the principal plane. A first conduction type barrier layer 107, an active layer 102, and a second conduction type barrier layer 108 are formed in sequence on the substrate 101. Then, a reflection mirror 103 made of a circular disk type multi-layered film is formed on the second conduction type barrier layer 108. With the use of the reflection mirror 103 as one reflection surface and a bottom surface of the barrier layer 107, i.e., a boundary surface between the barrier layer 107 and the substrate 101 as another reflection surface, an optical resonator having the reflection surface parallel with the active layer 102 and the resonance axis perpendicular to the active layer 102 can be obtained.
In the surface emitting type semiconductor laser which has the active layer formed of the zincblende crystal and is available at present, the reflection mirrors 103 have a multi-layered structure of semiconductor and are provided on both reflection surfaces of the optical resonator.
Conversely, the surface emitting type semiconductor laser using GaN, which is large in optical gain, as the active layer makes it possible to oscillate the light by using the reflection mirror 103 provided on one surface. The oscillation light is transmitted from an optical window which is formed on the lower face (opposite face to the principal plane) of the substrate 101. In the surface emitting type semiconductor laser, the active layer 102 is deposited eptaxially on the substrate 101. Accordingly, the active layer 102 made of hexagonal or zincblende crystal is formed as a thin layer in which the c axis of the hexagonal crystal or &lt;001&gt; axis of the zincblende crystal is perpendicular to the face.
On the other hand, since the resonator has the resonance axis perpendicular to the active layer 102, the oscillation light travels perpendicularly to the active layer 102 and the face of the active layer 102 serves as a polarization defining face. In other words, if an xy face is assumed to the polarization defining face and travel direction of the light is assumed to the z axis, the oscillation light consists of x polarized light 105 whose polarozation plane includes the x axis and y polarized light 106 whose polarozation plane includes the y axis. Since the z axis is perpendicular to the active layer 2, i.e., is parallel with the c axis of the hexagonal crystal or the &lt;001&gt; axis of the zincblende crystal and constitutes an axis of rotational symmetry in optical anisotropy, the x polarized light 105 and the y polarized light 106 have equal optical coupling for the crystal. Therefore, since the x polarized light 105 and the y polarized light 106 can be oscillated crystallographically at equal intensity, the polarozation plane of the oscillation light is not fixed and thus the polarozation plane becomes unstable.
In order to stabilize the polarozation plane of the oscillation light, such a semiconductor laser has been developed that only one of the x polarized light 105 and the y polarized light 106 can be oscillated by forming the reflection mirror as an elliptic or rectangular shape. However, this semiconductor laser has problems such that the emitted light cannot be formed as a circular beam and that it is hard to manufacture the shape of the reflection mirror precisely.
Further, in order to overcome such disadvantages, a semiconductor device has been proposed wherein symmetry of refractive index around the z axis is degraded by forming in-plane refractive index distribution in the active layer 102 or the barrier layers 107, 108 to thus define the polarozation plane of the oscillation light. However, since additional steps to form the refractive index distribution are needed, such situation is unavoidable that manufacturing steps is made complicated.
In the case of the conventional GaN system semiconductor, since highest energy bands HH, LH in valence band are degenerated doubly, hols are distributed in both HH and LH in this GaN system semiconductor. As a result, there has been the problem that threshold current density to cause laser oscillation is enhanced.
Upon growing the GaN system semiconductor on the 6H--SiC substrate, cracks easily occur on (0001) plane of the GaN system semiconductor layer due to thermal expansion stress and therefore it is difficult to obtain good crystal quality.
Then, in case compressive stress due to difference in thermal expansion coefficient and lattice mismatching is imposed on the Ga.sub.0.9 In.sub.0.1 N active layers 145, 155 constituting the light emitting portion in (001) face, as shown in FIG. 25B, energy in the CH band transfers from non-distortion state to a band structure which is relatively lower than HH, LH bands, but highest energy bands HH, LH in valence band are still degenerated doubly.
In addition, as another cause for higher threshold current density, it may also be considered that the sapphire substrate ha no cleavability.
As other problem in addition to the above problems in the conventional surface emitting type semiconductor laser having the vertical resonator, the polarozation plane of the oscillation light cannot be fixed because of small in-plane anisotropy of the active layer so that the polarozation plane of the emitted light cannot be fixed or oscillation becomes unstable.
In the structure in which the polarozation plane is specified by forming the reflection mirror as the rectangular shape, there have been the problems that the emitted light does not form the circular beam and the rectangular pattern is hard to be formed with the progress of miniaturization. In addition, in the case where in-plane refractive index is formed in the active layer or the barrier layer, simplification in manufacturing steps is limited.
If the surface emitting type semiconductor laser is employed as the light source of the magneto-optic disk drive, different problem arises due to undefined polarozation plane of the oscillation light.
More particularly, in the magneto-optic disk drive, uniform light polarization planes of the laser light are required since data are read by detecting rotation of light polarization plane of the light. However, since the light polarization plane is not made uniform in the conventional surface emitting type semiconductor laser, such laser has not been employed as a data reading device.